Printing of anodized aluminum

ABSTRACT

A PROCESS FOR DECORATING ANODIZED ALUMINUM INCLUDING PRINTING DRY POROUS ANODIZED SURFACE BY LITHOGRAPHIC OR GRAVURE MEANS WITH AN INK CONTAINING A VAPORIZABLE COLORANT, HEATING FOR AT LEAST 30 SECONDS TO INTENSIFY THE PRINT AND SEALING IN A HOT OR BOILING AQUEOUS SOLUTION OF A NEUTRAL SALT.

United States Patent Ofice 3,563,865 Patented Feb. 16, 1971 3 563 865 PRINTING F ANonIzED ALUMINUM Carl B. Blake, Bronx, and Adolph Fuerst, Brooklyn, N.Y.,

assignors to Martin Marietta Corporation, New York,

N.Y., a corporation of Maryland No Drawing. Filed Mar. 8, 1968, Ser. No. 711,505

Int. Cl. C23b 5/50 US. Cl. 204-35 6 Claims ABSTRACT OF THE DISCLOSURE When anodized aluminum is to be decorated with color, it is often done by dyeing the anodized metal in an aqueous bath of suitable dye. This method is particularly suited for overall uniform coloring. If prints or designs are to be made by dip dyeing, one must use a laborious masking technique. In general, such a masking technique would involve masking oif certain portions of the anodized aluminum surface, dip dyeing the unmasked portions, sealing those portions which are dyed, removing the masking and repeating the dip dyeing process one or a number of times. Since such processes are described in the literature including no less than 16 or 18 steps, it is obvious that except in unusual circumstances multicoloring of anodized aluminum by such a laborious process would be highly uneconomic.

It is possible to print anodized aluminum surfaces. Substantially all printing of anodized aluminum surfaces which has been done heretofore has been accomplished by silk screen methods. While silk screen printing is a time-honored technique, it is not usually capable of the fine detail which can be produced by modern gravure and offset processes. The silk screen process applies a relatively thick layer of ink, the very thickness of which tends to obscure details of the print, which details are crystal clear when gravure or offset processes are used. Another disadvantage of the silk screen method is that it is essentially a sheet fed operation whether it is used on anodized aluminum or on more conventional paper or paper board surfaces. Thus silk screen printing is not readily adaptable to continuous line coil fed operations.

It is the purpose and object of the present invention to provide a process wherein anodized aluminum can be printed in one or a plurality of colors by rapid offset or gravure techniques which are adaptable to high speed coil fed operations. Relatively recently aluminum has been anodized in a continuous manner. Metal in the form of sheet, strip or thick foil has been fed from large coils by means of a series of guides through anodizing, washing and sealing baths. As is well known in the industry, anodizing comprises making the aluminum surface an anode in an electrolyte and passing sufiicient electric current therethrough to form on the surface of the metal an oxidic layer which, in the case of a sulphuric acid electrolyte operated at ambient room temperatures, is

a porous. conventionally, the metal from the coil is led from the anodizing bath into one or more rinsing baths and then into a sealing bath" which usually comprises yeither boiling water or a very dilute aqueous solution of cobalt or nickel acetate held at or near its boiling point. When the anodized aluminum leaves the rinsing bath it is in a form such that the anodized surface is capable of absorbing dyes in the pores thereof. Conventional aqueous dip dyeing may be carried out at this point.

If, in addition to rinsing, one dries the rinsed anodized surface, the surface is then capable of absorbing dyes from organic solutions. Thus a printing operation, as contemplated in the present invention, is adaptable to present-day coil anodizing operations.

'It is an object of the present invention to provide a novel process of decorating anodized aluminum.

A further object of the present invention is to provide a novel process for multi-color printing of anodized aluminum.

Other objects and advantages will become apparent from the present specification.

Generally speaking, the present invention contemplates passing a coil fed sheet, strip or thick foil, i.e. an aluminum substrate, subsequent to anodizing, rinsing and drying and prior to sealing, through a printing station wherein the anodized aluminum substrate is brought into contact with a substantially planar surface print-defining roll having charged on the surface thereof in design configuration an ink comprising essentially a vaporizable organic colorant dissolved or dispersed in an organic binder solution, heating the thus printed surface to intensify the coloration thereof and sealing the thus printed surface in an aqueous bath containing a neutral salt selected from the group consisting of sodium sulphate, potassium sulphate, sodium chloride, potassium chloride and sodium chlorate. It is within the contemplation of the present invention to provide contact between the anodized aluminum substrate and more than one print-defining roller to provide plural and Inulti-colored prints on said anodized aluminum substrate before the heating and sealing operations.

In general, printing in the present process can be done by gravure or offset techniques. Briefly, gravure printing is accomplished by employing a printing element having ink-receiving cells therein. Ink is applied to the full surface of the element at one point, doctored off, except for that portion in the cells at a second point, and applied to the print-receiving anodized aluminum surface at a third point. Normally in coil-fed operations, the printing element is cylindrical and the three stations (inking, doctoring and printing) are angularly spaced apart from each other and abut on the periphery of the roll as it rotates in journal bearings. Offset printing uses two cylinders, the first of which, i.e. the plate cylinder, has separate water receptive and ink receptive areas on its surface. The ink receptive areas define the matter to be printed. The plate cylinder is wetted with an aqueous solution and then inked. The ink image is then transferred to a rubber blanket cylinder which in turn transfers the image to the anodized aluminum. Both the offset and gravure processes are characterized by printing a thin-film image having a thickness of less than about 2 mils, for example, a thickness of about 0.1 to about 0.8 mil. Consequently, inks used in these processes must be characterized by high tinctorial strength when compared to silk screen inks capable of printing the same color.

The aluminum used in the present invention can be any commercially pure aluminum or aluminum alloy which is normally used in industry for parts and structures comprising enclosures, cases, surfaces and the like which normally are desired to be decorated. Such aluminum (including alloys rich in aluminum) can be anodized by any conventional means which will produce a porous oxidic surface at a reasonable current density, e.g., 10 to 20 amperes per square foot. Anodizing can be successfully conducted in electrolytes such as dilute aqueous sulphuric acid, aqueous solutions of oxalic acid, aqueous chromic acid solutions and the like. One should avoid anodizing in a non-solvent electrolyte, such as aqueous solutions of boric acid, dibasic phosphates and the like, since the surface film produced by such anodizing is usually not sufficiently porous to permit colorant penetration. Specifically, it has been found that if aluminum is anodized at a current density of about 12 amperes per square foot in an aqueous solution containing about 12% to 15% by weight sulphuric acid maintained at a temperature of about C. (room temperature) a very satisfactory anodic film will be formed in anywhere from about 10 to about 40 minutes. Advantageously, the porous anodic film produced by anodizing will be about 0.2-0.6 mil thick. For convenience and economy, an anodic film about 0.3 mil is considered highly advantageous. After anodizing is complete, the anodized surface should be rinsed in water and allowed to dry. Decorating by printing by either gravure or offset methods is advantageously conducted within a short time after anodizing since exposure of a freshly anodized surface to moist air comprises a natural slow sealing process ultimately resulting in a sealed surface which will not accept colorations.

Advantageously the inks employed in the present invention contain a resinous binder, a solvent, and a vaporizable coloring agent (colorant), e.g., a pigment, a dye or the like. In general, the vaporizable coloring agent will be any colorant which, when heated in a thin film layer to a temperature of about 225 F. to about 500 R, will visibly sublime or otherwise volatilize. Unfortunately, there seems to be little or no means by which one can predict from its chemical structure whether a given colorant will volatilize under such conditions. In most instances, this characteristic can be best determined by experimentation. In order to give those skilled in the art a more complete understanding of the type of colorants which are contained in inks used in the process of the present invention, a substantial number of colorants are set forth in Table I, each of which is operable in inks in the present invention. These colorants are indicated by either a chemical name or by a trade name including indication, where possible, of the commercial source of the colorant and, where it is available, the designation in the Colour Index. It is to be appreciated that many of the colorants designated by trade names do not consist of pure chemical compounds and in many instances the exact structural configurations of the major compounds in commercially available colorants are not available.

TABLE I Colorant designation: Colour index designation Purpurin Mordant Dye (58205). Orasol Yellow 3G (CIBA) Solvent Yellow 17 (12770). Sudan Yellow GR (G) Solvent Yellow (21240). Oil Yellow SIS (CCC) Solvent Yellow 33 (47000). Oracet Red B (CIBA) Solvent Red 16. Oracet Violet B (CIBA) Solvent Violet 12 (61.105). Oracet Blue B (CIBA) Solvent Blue 19. Oil Base NS (CCC) Solvent Blue 61. Laytl Cerise B (Du P) Disperse Red 59.

Letters in parentheses in Table I (colorant designation) and elsewhere in this specification indicate a commerclal source of the colorant as follows:

CCC=American Cyanamid Co., Dyes Dept.

CIBA=Ciba Chemical and Dye Co., Div. of Ciba Corp. Du P=E. I. du Pont de Nemours & Co., Inc.

G=General Aniline & Film Corp., Dyestuff & Chemical Div.

The vehicle and solvent in the lithographic and gravure inks used in the present invention can be any of the vehicles and solvents normally employed in conventional gravure and lithographic inks provided that the vehicle does not carbonize under the heating contemplated by the invention. It is well known to those skilled in the art,

4 gravure and lithographic inks must have certain rheological characteristics which are attained principally by control of a vehicle and solvent content. The inks used in the process of the present invention do not differ from the conventional inks in these respects.

After the aluminum is anodized, rinsed and dried and is printed, the printed substrate, e.g., sheet, strip, or the like, is heated at a temperature of about 300-450 F. for a time ranging from about 30 seconds to 5 minutes. This heating can be conveniently conducted in a hot air oven through which the coil-fed strip or sheet runs directly from the printing press. It is to be observed that the printing press can be set up to print not only a single color but also to print a plurality of colors. As a result of the heating operation, the image on the printed substrate is intensified and driven more perfectly into the porous anodized film layer on the surface of the printed substrate. If desired, the heating step can be conducted at a lower temperature for a longer time. For example, temperatures as low as 200 F. can be used by loosely coiling the printed substrate and using a batch type oven for heating purposes. In such instances, the time of heating can be as much as 30 minutes or an hour. While heating times can be short at high temperatures, e.g., temperatures approaching 450 F. and higher, use of such temperatures is not normally recommended because anodized aluminum is subject to heat crazing upon exposure to such high temperatures. Advantageously temperatures of about 300 F. to 400 F. are used together with heating times of five to ten minutes.

After the heating step has been concluded and the image satisfactorily intensified, the substrate is sealed by means of an aqueous bath containing about 20% by weight sodium sulphate, said bath being held at approximately the boiling point. While, as mentioned hereinbefore,

salts other than sodium sulphate can be used, the economic factors are very favorable to the use of sodium sulphate. The salt is cheap and readily available. Other salts such as potassium sulphate, potassium chloride, and sodium chlorate are more expensive than sodium sulphate and do no seem to offer any particular advantage with respect thereto. The use of sodium chloride and potassium chloride entails some risk of corrosion if the sealing baths become contaminated with acid. While the sealing is advantageously conducted at the boiling point of the aqueous salt solution, temperatures as low as about F. can be employed. Sealing is conducted for at least about 5 minutes and advantageously from 20 to 30 minutes. While longer times can be employed, for example, up to an hour or two, no advantage is obtained by sealing for any amount of time beyond that which results in substantially complete closing of pores of the porous surface. After sealing is completed, the surface can be cleaned, if necessary, using an organic solvent such as methyl ethyl ketone and/ or the surface can be burnished or waxed in accordance with practices well known to those skilled in the anodizing art.

In order to give those skilled in the art a greater appreciation of the art and a better understanding of the invention, the following examples are given:

EXAMPLE I Aluminum sheet anodized in a dilute sulphuric acid bath to provide an anodic film thickness of about 0.4 mil is rinsed and dried. After printing with lithographic inks containing an oleoresinous binder, a solvent and colorants as set forth in Table II, the anodized aluminum samples are heated for 3 minutes at 375 F. and sealed in a boiling aqueous solution containing 20% by weight of sodium sulphate. The final prints are deep lustrous colors as specified in Table II and do not show any bleed onto the background.

TABLE II Colorant Colour index designation Color Sudan Yellow GR Solvent Yellow 30 (21240) Yellow. Oracet Red B Solvent Red 16 Red. Oil Blue NS Solvent Blue 19 Blue.

EXAMPLE II An anodized aluminum sheet as employed in Example I is printed by gravure in three colors using gravure ink bases containing the three colorants set forth in Table II. After heating for 2 minutes at about 300 F. and sealing in a hot, saturated, neutral sodium chloride aqueous solution maintained at about 200 F., the resulting print is clear with no trace of bleed either onto the background of onto non-overlap areas of diifering color print.

EXAMPLE III Results to those obtained in Examples I and II (except for the variance in color) are obtained using aluminum having anodic film thicknesses of 0.2 mil, 0.4 mil and 0.6 mil, heating at 250 F. for 30 minutes, sealing in aqueous boiling 20% sodium sulphate solution and employing the colorants set forth in Table III.

TABLE III Colorant Colour index designation Color Purpurin Mordant Dye (58205) Red. Orasol Yellow 3G Solvent Yellow 17 (12770) Yellow. Oracet Blue B Solvent Blue 19 Blue. Latyl Cerise B Disperse Red 59 Red.

on the outer surface of a porous, anodized aluminum substrate, said ink containing a vaporizable organic colorant, heating the thus printed anodic surface for at least about one-half minute at a temperature of at least 200 F., longer times being used with lower temperatures and vice versa, to drive said colorant into the porous surface and thereafter sealing the thus printed porous anodic aluminous surface in a hot aqueous solution of a neutral salt.

2. A process as in claim 1 wherein the printing is done by lithographic technique.

3. A process as in claim 1 wherein the printing is done by gravure technique.

4. A process as in claim 1 wherein the heating is conducted for about 30 seconds to about minutes at a temperature of about 200 F. to about 450 F longer times being used with lower temperatures and vice versa.

5. A process as in claim 1 wherein the neutral salt is from the group of sodium and potassium sulphates, sodium and potassium chlorides and sodium chlorate.

6. A process as in claim 1 wherein the sealing is conducted in a boiling aqueous solution containing about 20% sodium sulphate by weight.

References Cited UNITED STATES PATENTS 2,778,790 1/1957 Sobol 204-58 FOREIGN PATENTS 137,036 6/ 1961 U.|S.S.R. 445,242 4/ 1936 Great Britain. 1,380,091 10/ 1964 France.

OTHER REFERENCES Finishing of Aluminum, 2nd edition, Wernick et al., Draper Ltd., 1959, page 386.

JOHN H. MACK, Primary Examiner A. C. PRESCOTT, Assistant Examiner 

